Hi everyone, I was brokering this unusual for a client today and I sold it for exactly 15 pure! https://gyazo.com/65985c59ebf57930befe7596072f83f2

This suggestion seems accurate.

I sold mine for 20 keys pure not too long ago:

http://www.tf2outpost.com/trade/28744086 - B/O of 20

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1485907200/1485993600

Buyer: http://backpack.tf/profiles/76561198131703086#!/compare/1485734400/1485993600

http://imgur.com/a/ox0Hc

X = 20

Bought it for 15 via my buy order:

Seller: http://backpack.tf/profiles/76561198088043660#!/compare/1484179200/1484352000

Buyer: http://backpack.tf/profiles/76561198021055079#!/compare/1484265600/1484352000

X = 15

Though, given the number of sales lower than 20, it would probably be fair to keep 15 in the range.